Six situations and solutions that cause the switching power supply to whistle

The switching power supply controls the ratio of the turn-on and turn-off time of the switch in the circuit, maintaining a stable circuit voltage output, which is a very common power supply design. However, those who have worked on switching power supply design know that during the test of the switching power supply, some howling sounds are often heard, similar to the leakage sound emitted when the high voltage is bad, or the sound like a high voltage arc. So how do you solve these phenomena when they happen?

Generally speaking, the causes of switching power supply whistling generally have the following incentives.

1, PWM IC grounding line error

Usually, the product shows that some of the products can work normally, but some products cannot be loaded and may not be able to start the vibration. Especially when applying some low-power ICs, it is more likely that it will not work properly. For example, the SG6848 test board, because it did not thoroughly understand the performance of the IC, was rushed to lay out with experience, and the test could not do wide voltage test.

2, poor transformer dipping

Includes not impregnated with varnish. Howling and causing the waveform to have spikes, but the general loading capacity is normal, especially the description: the greater the output power, the stronger the howling, and the performance of the small power is not necessarily obvious. There is a bad load experience in a 72W charger product, and in this product, it is found that the material of the magnetic core has strict requirements. In addition, when the design of the transformer is not good, it is also possible that the vibration will be abnormal when working.

3, optocoupler working current point routing error

When the position of the optocoupler's operating current resistor is connected before the secondary filter capacitor, there is also the possibility of howling, especially when the load is more.

4, the reference voltage regulator IC TL431 grounding line error

The grounding of the same secondary reference regulator IC has the same requirements as the grounding of the primary IC, that is, it cannot be directly connected to the cold geothermal ground of the transformer. If connected together, the load capacity is reduced and the howling is proportional to the output power.

When the output load is large and close to the power supply limit, the switching transformer may enter an unstable state. In the previous cycle, the duty cycle of the switching tube is too large, the conduction time is too long, and excessive energy is transmitted through the high-frequency transformer; the energy storage inductor of the DC rectification is not fully released in the current period, and is judged by the PWM in the next cycle. There is no drive signal that causes the switch to conduct, or the duty cycle is too small. The switch is turned off during the entire period after the switch, or the on-time is too short. The energy storage inductor is released after more than one full cycle of energy, the output voltage drops, and the duty cycle in the next cycle of the switch tube is larger... so that the transformer has a lower frequency (regular intermittent full cutoff) The vibration of the cycle, or the frequency at which the duty cycle changes drastically, emits a lower frequency sound that the human ear can hear.

At the same time, the output voltage fluctuations will increase compared to normal operation. When the number of intermittent full cut-off cycles per unit time reaches a considerable proportion of the total number of cycles, the vibration frequency of the transformer originally working in the ultrasonic frequency band is reduced, and the frequency range of the human ear is audible, and a sharp high-frequency is emitted. The whistle is called." At this time, the switching transformer works in a severe overload state, and there is always the possibility of burning - this is the origin of many "screams" before the power supply burns. I believe that some users have had similar experiences.

5, when the load is empty or the load is very light

In this case, the switch tube may also have an intermittent full cut-off period, and the switching transformer also works in an overload state, which is also very dangerous. This problem can be solved by presetting the dummy load at the output, but it still happens occasionally in some "saving" or high-power power supplies.

6, when not loaded or the load is too light

The back EMF generated by the transformer during operation is not well absorbed. This way the transformer will couple many of the clutter signals to the windings. This clutter signal includes the AC components of many different spectra. There are also many low-frequency waves. When the low-frequency wave is consistent with the natural oscillation frequency of your transformer, the circuit will form a low-frequency self-excitation. The core of the transformer does not make a sound. We know that the human hearing range is 20-20KHZ. So when we design the circuit, we usually add a frequency selection loop. To filter out low frequency components. It is best to add a bandpass circuit to the feedback loop to prevent low frequency self-excitation. Or you can make the switching power supply a fixed frequency.

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